In resistance welding a considerable current flows through the superimposed plates or parts, which are compressed between two electrodes or tool parts. The contact resistance between the plates, generally made from steel, is considerable. The current flow brings about a heating, which once again leads to the formation of a core of melted metal from both plates, so that the desired connection is obtained on cooling. The energy required for forming the melting core is rI.sup.2 t, r; the sum of electrical resistances, I=r.m.s. current and t=the total current flow time.
In order to achieve high quality resistance welding, it is necessary to obtain a very specific product rI.sup.2 t. As the resistance, essentially determined by the contact resistances, is dependent on the parts to be connected or joined together, the basic parameters for the welding control or regulation are the welding time and intensity, i.e. the r.m.s. welding current.
Welding control systems with automatic welding time variation are known. However, the conditions for automatic production require minimum and, in particular, precise times for the working cycles, which requires forecastable welding times and therefore solutions with current control or regulation at preset time are preferred.
Known resistance welding machines have an electric primary supply with e.g. 220 or 380 V a.c. voltage. They are provided with a thyristor interrupter formed from two oppositely directed thyristors, whose control inputs are controlled by control electronics. The interrupter supplies the primary coil of a welding transformer, whose secondary coil, which generally supplies a voltage of 3 to 20 V, is connected to arms or welding machine electrodes fixed thereto. The current flows across the electrodes and thus brings about the welding of the plates located between the same. For this purpose the electrodes are pressed, generally pneumatically, on to the plates with a predetermined force, which is generally several hundred decaNewtons.
Two processes are known for controlling the welding current. Both processes make it possible to work with a predetermined constant welding time. In welding practice, the welding time is generally expressed as the number of periods or cycles of the electric current or voltage. For example, welding for ten periods at 50 Hz supply voltage corresponds to a welding time of 0.2 sec. In all known processes with thyristor control for regulating the current, the firing angle or phase of the thyristor is controlled.
A simple control of the welding current makes it possible to program the number of alternating current periods during which a flow of current through the weld point is allowed and a firing angle .psi. of the thyristor at each polarization change, which makes it possible to adjust the r.m.s. value of the welding intensity. Thus, thyristor firing takes place at an angle .psi. following the voltage zero passage and the current intensity only reaches zero at an angle .phi. following a further zero passage. The angle .psi. serves as a control value, whereas the angle .phi. is obtained on the basis of the reactance of the electric circuit of the machine.
This known current control by varying the firing angle of the thyristors controls the average current or r.m.s. current during a period, which is determinative for the welding.
This open system, without any measurement of the actual current flow and without regulation, does not guarantee a constant welding intensity either during spot welding, or from one spot weld to the next. Thus, there can be variations in the ratio U/Z=I (U=r.m.s. value, I=r.m.s. current, Z=impedance) due to fluctuations in the primary voltage or variations in the total impedance of the installation.
Therefore a process has already been proposed in which the welding current is regulated. For this purpose the r.m.s. welding intensity is measured during each period and compared with a desired value. From this comparison an analog or digital computer determines the desired thyristor firing angle for the next period, so that during the latter a welding intensity equal to the predetermined desired intensity is obtained. An apparatus for performing this process has a computer for determining the displacement or shift of the firing angle, as well as a coil and a following integrating unit for measuring the actual welding current. This process makes it possible to take account during the follow-up period n+1, voltage and impedance changes during a first welding period.
For example, during first welding period, the current can be much lower than the desired value, which is often the case at the start of new welding, because the system has no reference information for the angle .psi.. The following periods can then be regulated in the described manner, so that, to the extent that, for example, the power supply remains constant, the actual current flowing corresponds to the desired value. However, if the voltage drops, for example, due to simultaneous welding of another machine connected to the same network, a lower current flow occurs and account is taken thereof in the further periods as a result of the regulation, so that during the same the current flow corresponds to the desired value. Thus, the average intensity occurring during welding even in the case of perfect operation of the regulation system is lower than the desired value, so that the weld point does not have the desired, intended characteristics.